The effect of sudden changes in temperature, light and salinity on the population density and export of zooxanthellae from the reef corals Stylophora pistillata Esper and Seriatopora hystrix Dana

Abstract

Bleaching (loss of pigmentation by corals) is a widespread phenomenon in coral-reef ecosystems. Despite this, the underlying causes of some forms of bleaching are poorly understood. This study explores the conditions that induce bleaching in two species of reef coral-zooxanthellae associations from Lizard Island, Great Barrier Reef, Australia. Naturally bleached Stylophora pistillata Esper and Seriatopora hystrix Dana, collected from the edge of Lizard Island lagoon, had the same amount of Chl a · zooxanthellae −1, yet had reduced population densities of zooxanthellae when compared to normal-looking colonies. In this case, the lack of pigment in the bleached corals was explained by low numbers of zooxanthellae and not by pale zooxanthellae. This is contrary to results obtained by some other workers and suggests that closer inspection of the underlying reasons for the pale color of bleached corals is warranted. In laboratory experiments, sudden exposures to full sunlight induced the bleaching of S. pistillata previously grown at 25% sunlight. The pale color of colonies exposed to full sunlight was explained by the low pigment content of the zooxanthellae rather than by low population densities of zooxanthellae. In addition, the specific expulsion rate of zooxanthellae from S. Pistillata and S. hystrix was not influenced by sudden increases in solar irradiance. Sudden exposures to reduced salinities (30%.) did not affect S. Pistillata or S. hystrix in this study. Both species bleached rapidly, however, when exposed to water temperatures of > 30 °C. Bleached corals in this case had reduced population densities of zooxanthellae despite normal zooxanthella pigment contents. The specific expulsion rate of zooxanthellae from both species of coral was very sensitive to temperature. 7-h exposures to 30 and 32 °C resulted in specific expulsion rates of > 1000 times that of controls. Specific expulsion rates remained high, even when corals were returned to control temperatures (27 °C). After high temperature stress, S. Pistillata and S. hystrix had high and sustained colony respiratory rates ( r c ), reduced colony photosynthetic rates ( p c net max ) and reduced P c g max : r c ratios. The O 2 metabolism of the S. pistillata and S. hystrix symbioses remained abnormal for up to 4 days after a 7-h exposure to 32 °C. Evidence suggestive of recovery was apparent 19 days later. By this time, the mean population density of zooxanthellae and p c g max : r c ratios of exposed fragments had returned to normal.